1. Field of the Invention
The present invention relates to a method or process for preparing aromatic ketones by a ketonic decarboxylative coupling of aromatic carboxylic acids. More particularly, the present invention relates to a process for preparing benzophenones by means of a gas-phase or vapor-phase coupling of aromatic carboxylic acids.
2. Description of the Prior Art
It has been shown that ketones can be formed by means of ketonic decarboxylation of carboxylic acids. For example, in an article in ZH. OBSHCH. KHIM., 30, 9, 2789 (1960), Rubinshtein et al., discussed the use of ThO.sub.2, CeO.sub.2, CaCO.sub.3, BaCO.sub.3, ZnO.sub.2, and CdO as active catalysts for ketonization and the vapor-phase catalytic ketonization of acetic acid over carbonates of alkaline earth metals (Ca, Ba, Sr, and Mg). In KINET. KATAL., 2, 2, 172 (1961), Yakerson et al., investigated the kinetics of the thermal decomposition of lithium, sodium, and barium acetate to ketone and used the data to specify the mechanism of the vapor-phase ketonization of acetic acid and its decomposition to methane. In KINET. KATAL., 2, 6, 907 (1961), Yakerson et al., discussed the kinetics of vapor-phase catalytic ketonization of acetic acid over TiO.sub.2, ZrO.sub.2, SnO.sub.2, CeO.sub.2, and BeO. In IZV. AKAD. NAUK SSSR, No. 1, 83 (1966), Yakerson et al., discussed the catalytic ketonization of acetic acid over a mixed binary catalyst system of ZrO.sub.2 -Al.sub.2 O.sub.3. In ZH. PRIKL. KHIM., 50, 2126 (1977), Shmelev et al., reported that diethyl ketone could be prepared by the ketonization of propionic acid in the presence of a catalyst of manganese dioxide supported on silica gel.
Furthermore, in Japanese Published patent application Kokai No. Sho 57(1982)-197237, Matsuoka disclosed a method for preparing ketones from straight-chain or branched aliphatic carboxylic acids by employing a gas-phase contact reaction whereby an aliphatic carboxylic acid is contacted with a catalyst comprising zirconium oxide and, optionally, a support of alumina or silica gel. In this Japanese patent publication, he also listed conventionally-used catalysts for the synthesis of a molecule of ketone from two molecules of a carboxylic acid as being calcium oxide, barium oxide, lithium oxide, alumina, chromium oxide, manganese oxide, thorium oxide, gallium oxide, indium oxide, and oxides of rare earth elements and mentioned that such catalysts provide low conversions and selectivities.
In U.S. Pat. No. 4,014,889, Schreckenberg et al., disclosed the preparation of a ketone by means of the reaction of an aromatic or heterocyclic aldehyde in the presence of a cyanide ion with an unsaturated compound having the formula: EQU R.sub.1 -C(R.sub.2)=C(R.sub.3)-R.sub.4,
"R.sub.1," "R.sub.2," and "R.sub.3 " are the same or different and are selected from the group of hydrogen, optionally substituted aliphatic, cycloaliphatic, araliphatic, aromatic, heterocyclic and carboxylic ester and "R.sub.4 " is nitrile, --CO--R.sub.5 or --CO--OR.sub.5 wherein "R.sub.5 " is selected from the group of optionally substituted aliphatic, cycloaliphatic, araliphatic, aromatic, and heterocyclic and "R.sub.1 " and "R.sub.2 " and/or "R.sub.1 " and "R.sub.3 " and/or "R.sub.2 " and "R.sub.5 " or "R.sub.3 " and "R.sub.5 " together with the carbon atoms to which they are attached as substituents can also form a carbocyclic or heterocyclic ring.
In U.S. Pat. No. 3,479,400, Lese et al., disclosed a process for converting a 1,1-diarylalkane to the corresponding diaryl ketone, which process involves oxidizing the 1,1-diarylalkane in a first reaction zone with nitric acid, reacting the solid product from the first reaction zone with nitric acid in a second reaction zone under conditions that are more severe than those employed in the first reaction zone to obtain the desired diaryl ketone and optionally recycling an aqueous solution containing nitric acid to the first reaction zone.
In U.S. Pat. No. 4,007,211, Trost et al., disclosed a method for converting an alpha-thiocarboxylic acid compound to the corresponding ketone. In this method, the carboxylic acid or its ester is first subjected to a sulfenylation reaction for positioning an "--SR" group alpha to the carboxylic acid group and then the sulfenylated product is subjected to oxidative decarboxylation in the presence of an alkali metal periodate or a halogen source.
It has now been found that aromatic ketones, such as benzophenones, can be prepared in improved yields by the ketonic decarboxylative coupling of aromatic carboxylic acids.